Edible oils as obtained by expelling and/or solvent extraction often require extensive purification. Accordingly, mucilaginous matter may be removed during a degumming step, and free fatty acids may be removed during a chemical neutralisation step by reacting them with caustic soda under formation of soaps. Colouring compounds may be removed during the a bleaching step wherein oil is treated with an adsorbent such as bleaching earth, and malodorous compounds may be removed during a deodorization step. This latter step can also serve the purpose of removing free fatty acids and is then commonly referred to as the steam refining or physical refining step.
For the removal of mucilaginous matter from a vegetable oil, a number of different processes are used. The most simple and oldest process is the water degumming process as described by Bolmann in German Patent 382 912. In this process, crude oil as obtained by solvent extraction of oilseeds or their expeller cake is treated with live steam after the extraction solvent has been removed by evaporation. The term “live steam” should be understood herein as meaning that steam is directed at the object being steamed and allowed to condense onto said object. This treatment causes the gums to be hydrated and form a separate phase that can then be isolated from the oil as wet gums which can then be dried to yield lecithin.
As explained in detail by W. van Nieuwenhuyzen, “Lecithin production and properties” in Journal of the American Oil Chemists' Society (1976) 53: 425-427, lecithin is a complex mixture of phosphatides (such as phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl inositol and phosphatidic acid), their lyso-compounds, triglycerides, sugars (such as saccharose, raffinose and stachyose), some glycolipids and further minor constituents, wherein lecithin normally contains some 35% by weight triglyceride oil.
This triglyceride oil contained in lecithin constitutes a manufacturing and/or commercial loss to the crushing industry (i.e. oil production from pressing oil seeds) since the addition of the lecithin or the wet gums to the meal only realises meal value, which is invariably lower than the oil value. Accordingly, attempts have been made to reduce this loss by recuperating oil from the wet gums or lecithin. U.S. Pat. No. 2,678,327 describes a process of treating crude glyceride oil containing gums, comprising de-gumming said oil by adding an aqueous de-gumming agent thereto to hydrate said gums and render them insoluble in said oil and separating from said oil hydrated gums containing a substantial amount of said oil, thereafter admixing a liquefied, normally gaseous hydrocarbon such as propane with the hydrated gums and maintaining the resulting mixture under sufficient pressure to retain said hydrocarbon in liquid form, bringing said mixture to a temperature producing a solvent phase containing said liquefied hydrocarbon and said oil and an aqueous phase containing said gums while said gums contain sufficient water to produce a liquid aqueous phase, separating said two phases, recovering de-oiled gums from said aqueous phase and vaporising said hydrocarbon from said solvent phase to recover oil therefrom.
The use of a normally gaseous hydrocarbon such as propane necessitates the installation to be explosion-proof which requires a considerable additional investment. Moreover, liquefaction and evaporation of this hydrocarbon require energy and thus augment the operating cost of the recovery process. Consequently, the crushing industry would welcome a cheaper process to recuperate triglyceride oil contained in wet gums.
During the water degumming process leading to the wet gums not all phosphatides present in the crude oil are hydrated. Some phosphatides that are commonly referred to as non-hydratable phosphatides (NHP), remain dissolved in the oil even after repeated water degumming treatments. Since these NHP have to be removed if the oil is subsequently to be neutralised by the steam refining process, more effective degumming processes have been developed that result in lower residual phosphatide contents than the water degumming process.
Several of these more effective degumming processes have in common that they treat the crude oil to be degummed with an aqueous acid in order to decompose the NHP which consist mainly of calcium and magnesium salts of phosphatidic acid. For this decomposition to be effective, the degumming acid has to be stronger than phosphatidic acid, and phosphoric acid or citric acid are commonly used for this purpose. U.S. Pat. No. 4,049,686 discloses that phosphatides and other minor components can be advantageously removed from crude or water-deslimed triglyceride oils which are substantially liquid at 40° C., by dispersing an effective amount of a substantially concentrated acid or anhydride, having a pH of at least 0.5 as measured at 20° C. in a one molar aqueous solution, in the oil, subsequently dispersing 0.2 to 5% by weight of water in the mixture obtained, and finally separating an aqueous sludge containing the gums from the oil, the mixture of oil, water and acid being maintained for at least 5 minutes at a temperature below 40° C. before separating the aqueous sludge.
European Patent No. 195,991 discloses producing de-gummed vegetable oils and gums of high phosphatidic acid content by removing non-hydratable phosphatides and iron from water de-gummed vegetable oils, comprising the steps of:                finely dispersing a non-toxic aqueous acid in the water de-gummed oil so as to form an interface between the acid droplets and the oil of at least 0.2 m2 per 100 g of oil, and allowing sufficient contact time to complete decomposition of the phosphatidic acid metal salts; and        mixing a base into the acid-in-oil dispersion in such amount that the pH of the aqueous phase is increased to above 2.5 but no substantial amount of soap is produced.        
Yet another acid degumming process has been described in U.S. Pat. No. 6,172,248. In this process, vegetable oil is combined with a dilute aqueous organic acid solution and subjected to high shear to finely disperse the acid solution in the oil. The resulting acid-and-oil mixture is then mixed at low shear for a sufficient length of time to sequester contaminants into a hydrated impurities phase, producing a purified vegetable oil phase. Example 1 of this patent illustrates that the resulting oil is indeed highly purified and differs strongly from conventionally water degummed vegetable oil. Whereas according to Table 1 in U.S. Pat. No. 6,172,248 the latter oil contains 50 ppm magnesium, 80 ppm calcium and 150 ppm phosphorus, the organic refined oil contains <5 ppm magnesium, <5 ppm calcium and <10 ppm phosphorus. This means that the composition of the hydrated gums resulting from the organic refining process will also differ substantially from that resulting from the conventional water degumming process. Whereas the latter comprises only hydratable phosphatides, the gums resulting from the organic refining process also contain the NHP or rather their decomposition products such as for instance the sodium salt of phosphatidic acid.
Finally, U.S. Pat. No. 6,426,423 discloses a process for treating a phosphatide-containing mixture that comprises the steps of providing a phosphatide-containing mixture obtained from an organic acid refining process; separating the phosphatide-containing mixture into a purified vegetable oil phase and a phosphatide-enriched aqueous phase; and removing the purified vegetable oil phase. Typically, the purified vegetable oil phase migrates to the top, the phosphatide phase migrates to the middle and the aqueous organic acid phase migrates to the bottom. The purified vegetable oil phase can be separated by any convenient method, although preferably it is decanted away from the other two phases.
Accordingly, the process described in U.S. Pat. No. 6,426,423 allows oil contained in wet gums to be recuperated and thus to reduce the oil loss during the crushing process, but it has the disadvantage of being limited to the organic refining process as described in U.S. Pat. No. 6,172,248 and employing additives like organic acids and also aiming at producing an oil with a low residual phosphorus content. This oil recuperation process is therefore of little use to the crushing industry who wants to supply the market with vegetable oils containing as much phosphatides as permitted by the trading specifications and who want to maximise their crude oil yield at the same time.
In the crushing industry, vegetable oils are commonly just water degummed to reduce the risk that the oils throw a deposit on transport and storage and to ensure that the refiner buying this crude oil suffers only limited refining losses. The gums resulting from the water degumming process may be dried to yield lecithin but since the potential availability of lecithin far exceeds the demand, most gums are mixed with the meal in the crushing plant and then sold at meal value, which is fully in line with existing regulations since this mixing is confined to an integrated operation. Moreover, the only lecithin being traded in appreciable amounts is soy lecithin which means that wet gums originating from the water degumming of for instance rapeseed oil or sunflower seed oil are hardly ever dried as such and thus normally mixed with their meals.